The cornea is the transparent hemispherical structure located in front of the eye, which allows the passage of light and protects the iris and lens. It has the shape of a concave disc with an average diameter of 11.5 mm in humans and it possesses significant refractive optic properties, providing approximately 70% of the total focusing ability of the eye.
The cornea consists of three layers: the outermost layer is the corneal epithelium composed of pluri-stratified non-keratinized epithelium with an enormous regenerative potential; the intermediate layer is the stroma, the widest layer of the three; and the mono-stratified endothelium is the innermost layer consisting of one single layer of cells. Two membranes separating the stroma from the other two corneal layers are distinguished: Descemet's membrane separating the stroma from the endothelium and Bowman's membrane separating the stroma from the epithelium.
The epithelium represents 10% of the total thickness of the cornea (approximately 550 microns in humans (and is formed by several layers of cells acting as a protective barrier against external agents. The transport of ions through cells of the epithelial layer is one of those responsible for regulating corneal functionality.
The stroma is formed in humans by 200 to 250 sheets of collagen fibers arranged parallel to the corneal surface. The stroma is ultimately responsible for the biomechanical properties of the cornea, as well as its curvature and transparency. To maintain its transparency it does not have capillaries for nourishing the cornea, such that nutrients are supplied through the epithelium and endothelium. On the other hand, the degree of stromal hydration is directly related to the transparency of the cornea, remaining in a constant state of dehydration.
The endothelium consists of a monolayer of cuboidal cells forming a hexagonal mosaic and maintains the transparency of the tissue by controlling stromal hydration. On one hand, there is a flow by diffusion between endothelial cells controlled by the junctions between them (tight junctions). On the other hand, endothelial cells are specialized in pumping water from the stroma to the aqueous humour, creating an active flow through the fluidic pump and ensuring the homeostasis of the cornea. A feature of the corneal endothelium, unlike the epithelium, is its inability for cell renewal. This causes a loss of cell population with age as well as a decrease in their thickness since the cells are forced to cover the entire posterior corneal surface, leading to a loss of its ability to control stromal hydration level as a result. This aging can occur in an exaggerated manner in dystrophies and as a result of disease, eye surgery or use of ophthalmic drugs.
FIG. 1 shows the different layers making up the cornea, as well as the main mechanisms for controlling the stromal hydration level: diffusion through the epithelium, diffusion and pumping through the endothelium. Although the study of the permeability of these layers is of great clinical interest, so far only studies based on in vitro measurements, usually with tissues removed and placed in a specific sensor device, have been conducted. There are also some studies that take in vivo measurements in animals, but using highly invasive methods, which usually involves the impairment of the tissue for further studies and the impossibility to do so in a clinic with patients. In clinical practice the measurement of the thickness of the cornea (pachymetry) is often used as an indirect measurement for corneal dysfunction.
The study of passive electrical properties of the different layers of the cornea is commonly used in in vitro studies to evaluate their permeability. But the methods used in these studies are not applicable to in vivo measurements. Cellular and acellular media have a different behavior to electric current. In general, the tissues are composed of cells embedded in an extracellular medium. At low frequencies, <1 kHz, the current is distributed in the extracellular medium (essentially an ionic solution with resistive behavior), while at higher frequencies, >100 kHz the current is capable of passing through cell walls and intracellular medium (the behavior of the membranes is capacitive and the intracellular medium is resistive). FIG. 2 graphically depicts this difference in behavior as a function of frequency. Based on this behavior of biological tissues the status of the different layers of the cornea can be analyzed using measurements based on its passive electrical properties, such as is the case of impedance measurements.